The present invention relates to a method for producing polyolefins, especially propylene block copolymers, and to the production method and a vapor-phase polymerization apparatus for it, in which olefins are polymerized in a mode of vapor-phase polymerization in the presence of an olefin polymerization catalyst, with preventing the polymers produced from depositing in reactors and preventing them from growing into abnormal agglomerates or aggregates, and which therefore ensure long-term continuous and stable production of high-quality polyolefins.
For the production of polyolefins such as polypropylene and propylene block copolymers, Ziegler-Natta catalysts have been improved to have high activity and high stereospecificity, and the polymer productivity per the unit catalyst used has been greatly increased with the increase in the stereospecificity of the polymers produced. As a result, it has become possible to reduce the metal content such as the catalyst-derived transition metal content of the polymers produced and to reduce the amorphous polypropylene content thereof. Accordingly, for the polymer production method, a mode of vapor-phase polymerization is now increasingly popular in place of conventional solution polymerization, slurry polymerization and bulk polymerization, as it does not require solvent recovery and purification, it facilitates monomer recovery and polymer drying, and it realizes product diversification.
For example, propylene block copolymers are produced in a two-stage process comprising producing a crystalline homopolymer or copolymer of propylene in the former stage polymerization reactor, followed by producing a rubber-like random copolymer of propylene and other xcex1-olefins such as ethylene in the latter stage polymerization reactor. The propylene block copolymer compositions produced have high mechanical strength, good toughness and good heat resistance intrinsic to the crystalline polypropylene, and have good impact resistance, especially good low-temperature impact resistance intrinsic to the rubber-like random copolymer. Therefore, they are widely used for automobile parts including outer members such as bumpers and inner members such as inner panels and doors, and for containers, sheets, etc.
Accordingly, the method of vapor-phase polyolefin production is an extremely excellent process. In the vapor-phase method, however, the polymer produced is separated into a powder phase and a vapor phase in the polymerization reactor of any type having a vapor-phase fluidized bed or an agitation fluidized bed, and therefore, the polymer in the reactor could not be fully fluidized, stirred and unified. Therefore, when compared with that produced in a method of solution polymerization or slurry polymerization, stirring and unifying the polymer produced in the vapor-phase polymerization method is often unsatisfactory. In particular, in the vapor-phase polymerization method of producing propylene block copolymers mentioned above, the rubber-like copolymer produced in the second-stage random copolymerization reactor is sticky, and the polymer and copolymer particles often agglomerate or aggregate and often deposit on the wall of the reactor and on the stirring blades in the reactor.
The polymer deposition interferes with stable, long-term continuous polymerization in the reactor, and, in addition, causes excessive increase in the molecular weight of the polymer produced, and, as the case may be, the polymer is often gelled in the reactor. As a result, the quality of final polymer moldings will be low. Still another problem is that small polymer agglomerates having deposited on the wall of the reactor often clog pipe lines of polymer powder. In addition, they will clog filters in monomer-cooling pipe lines. For these reasons, the quality of final polymer moldings is worsened to the following effect. The polymer having deposited on the wall of a reactor stays in the polymerization line for a long period of time and is gelled to be a non-melting or hardly melting matter. This worsens the appearance of the moldings of the polymer produced in the line, and will be the start point of the fracture of the moldings. After all, the physical properties and the commercial value of the moldings are much lowered.
Accordingly, polymer deposition and agglomeration must be prevented in the process of producing polyolefins, especially propylene block copolymers. For this, Japanese Patent Laid-Open Nos. 151713/1981 and 213012/1983 disclose a method of adding an alkoxyaluminium compound to the polymerization system. In this, however, the alkoxyaluminium compound is not effective if its amount added is not large. Since the aluminium content of the polymer produced therein increases, the method is unfavorable for vapor-phase polymerization.
Japanese Patent Laid-Open No. 69821/1986 discloses a method of using a high-stereospecificity polymerization catalyst, in which from 0.001 to 1 mol, per gram of aluminium of the catalyst, of an active hydrogen compound is fed into the random copolymerization system. However, the method disclosed is for batch polymerization, and not for continuous polymerization. The laid-open specification shows a concrete mode of feeding an active hydrogen compound to the random copolymerization system and says that the bulk density of the polymer produced is high. In this, however, nothing is referred to about the prevention of polymer deposition in reactors. This is natural since the batch polymerization employed therein produces uniform polymers.
On the other hand, Japanese Patent Laid-Open Nos. 225613/1988, 296313/1992, 296314/1992 and 71415/1999 relate to vapor-phase polymerization, and concretely disclose how to feed an alcohol into the monomer-cooling pipe line and into the polymer-transferring pipe line that connects a crystalline polypropylene-producing polymerization reactor and a propylene random copolymer-producing polymerization reactor. In these, however, the olefin polymerization catalyst activity retardant is not directly fed into the polymerization reactors. In the methods disclosed, the catalyst activity retardant is added to the polymerization system by feeding it into the polymer powder pipe line or into the monomer pipe line, to thereby improve the dispersibility of the polymer produced. Anyhow, in these, the catalyst activity retardant is finally added to the polymerization reactors, and the methods will be good.
In Japanese Patent Laid-Open No. 71415/1999, disclosed is an example of metering and feeding a heptane solution of 17 wt. % isopropanol into a pipe line that connects two reactors, former-stage and latter-stage reactors. In this, also disclosed is a comparative example of metering and feeding the same solution directly into the latter-stage reactor. In these example and comparative examples, the systems were driven for 3 weeks, and then compared with each other. They say that the amount of the agglomerates and the aggregates seen in the powder bed in the latter-stage reactor in the method of the example was reduced to 35 to 45% of that in the comparative example; and the amount of the films and the deposits on the reactor wall and on the baffles was reduced to 25 to 35% of that in the comparative example.
To that effect, the conventional improved methods could produce good results in some degree. In Japanese Patent Laid-Open No. 71415/1999, the effect of reducing the polymer agglomeration, aggregation and deposition is quantitatively evaluated. However, even though the method disclosed therein is improved in some degree, it is obvious that its effect of reducing the polymer agglomeration, aggregation and deposition is limited and is not always satisfactory. Therefore, depending on the type of the polymerization reactors used and the polymerization conditions employed, non-negligible polymer agglomerates and aggregates are formed in the method and they deposit on the reactor wall and on the stirring blades, thereby often clogging the polymer take-out pipes. The problem with the method must be solved.
The present invention is to solve the problems with the related art techniques as above, and its object is to provide a method for producing polyolefins through continuous vapor-phase polymerization in the presence of an olefin polymerization catalyst, and a vapor-phase polymerization apparatus for the method free from the problem of non-uniformity in vapor-phase polymerization. Specifically, in the vapor-phase polyolefin production method of the invention, the (co)polymer produced does not form agglomerates and aggregates, and does not deposit on the reactor wall and on the stirring blades, and abnormal reaction products and large particles are prevented from being formed therein. Therefore, in the method, the filters and the pipe lines are not clogged, and the method ensures stable, long-term continuous polymerization, with no abnormal polymerization to lower the quality of the polymers produced.
To attain the object as above, we, the present inventors have assiduously studied the relationship between the mode of feeding a catalyst activity retardant to the polymerization system and the polyolefin polymer deposition in reactors and around stirring blades and the formation of abnormal polymer agglomerates and aggregates. As a result, we have found that the mode of feeding a catalyst activity retardant to polymerization reactors has a significant influence on the polymer deposition in the reactors and around the stirring blades and also on the formation of abnormal polymer agglomerates and aggregates, thereby often lowering the polymer productivity and worsening the polymer product quality. On the basis of these findings, we have completed the present invention.
Specifically, the invention provides the following:
(1) A method for producing polyolefins through continuous vapor-phase polymerization in the presence of an olefin polymerization catalyst, which is characterized in that a catalyst activity retardant is fed into the vapor phase in the polymerization reactor and into the powder phase therein through its side wall.
(2) The method for producing polyolefins of above (1), wherein the catalyst activity retardant is fed into the powder phase through a plurality of openings in the side wall of the polymerization reactor.
(3) The method for producing polyolefins of above (1) or (2), wherein the plurality of openings in the side wall of the polymerization reactor through which the catalyst activity retardant is fed into the powder phase are both in the upper and lower parts of the reactor and are spaced from each other in the peripheral direction of the side wall.
(4) The method for producing polyolefins of any of above (1) to (3), wherein the amount of the catalyst activity retardant fed into the polymerization reactor falls between 0.001 and 5 g per kg of the polyolefin produced, and the ratio of the catalyst activity retardant fed into the vapor phase in the reactor to that into the powder phase therein falls between 95/5 and 10/90 by weight.
(5) The method for producing polyolefins of any of above (1) to (4), wherein the catalyst activity retardant is fed into the polymerization reactor along with a carrier fluid.
(6) The method for producing polyolefins of any of above (1) to (5), wherein the catalyst activity retardant is at least one selected from alcohols, phenols, carboxylic acids, sulfonic acids, amines, amides, esters, ethers, phosphines, water, carbon monoxide and carbon dioxide.
(7) The method for producing polyolefins of any of above (1) to (6), wherein the catalyst activity retardant is an active hydrogen-containing compound.
(8) The method for producing polyolefins of any of above (1) to (7), which comprises producing a crystalline polypropylene of either a propylene homopolymer or a propylene copolymer having an additional xcex1-olefin content of at most 5% by weight in a first polymerization reactor, and producing a propylene block copolymer through random copolymerization of propylene and an xcex1-olefin different from propylene in the presence of the crystalline polypropylene in a second polymerization reactor, and wherein the catalyst activity retardant is fed into the second polymerization reactor.
(9) A vapor-phase polymerization apparatus for polyolefins, which comprises a monomer supply line, a polymer take-out line, a monomer circulation line, and a polymerization reactor optionally with a stirrer therein, and which is equipped with a supply unit for feeding a catalyst activity retardant into the vapor phase in the polymerization reactor and into the powder phase therein through its side wall.
(10) The vapor-phase polymerization apparatus for polyolefins of above (9), wherein the catalyst activity retardant supply unit has a plurality of outlets that are directed to the powder phase in the polymerization reactor and are spaced from each other in the peripheral direction of the side wall of the reactor.